DO NOT MERGE Update libpng to 1.6.20 am: cf8a579007 am: a0b5142c6c am: 34dcc2af83  -s ours am: 3aec76d873  -s ours am: 9cac6eee10 am: c34faaeec7 am: 8ecc2c0512  -s ours am: 89d19aa8f1 am: 9a9e74b2ec am: 0f95eaef8e am: fd18529232  -s ours
am: aa0664c5f8

* commit 'aa0664c5f8c982896845eb73c4ae2c06ad3fe879':
  DO NOT MERGE Update libpng to 1.6.20

Change-Id: I432c14f6836d0fa0302ead6eb36696054f0bcced